Japanese Patent Application Publication No. 5-87760 (Patent Document 1) discloses a method by which a noble metal is mixed in a metal oxide precursor so as to manufacture a metal oxide semiconductor material for gas sensors. In such a metal oxide semiconductor material used for gas sensors, a noble metal colloid obtained by coating particles of a noble metal such as Pt or Pd with a polymer or the like and making them into a colloidal state is used to improve the dispersibility between the metal oxide precursor and the noble metal for the purpose of enhancing the sensor sensitivity of gas sensors. The noble metal colloid contains many impurities which may cause a decrease in the sensor sensitivity, and the impurities are mixed into a dispersion in which the metal oxide precursor and noble metal colloid are mixed and dispersed. Therefore, the dispersion of the metal oxide precursor and noble metal colloid is purified with ion-exchanged water so as to remove the impurities from the dispersion. In this case, the noble metal colloid has a low specific gravity and is thus not precipitated by spontaneous sedimentation. Then, after adding the ion-exchanged water to the dispersion of the metal oxide precursor and noble metal colloid for purifying, a centrifuge is used to forcibly separate the purified dispersion into a supernatant and a precipitate. Then, the supernatant containing impurities is removed to eliminate the impurities from the dispersion. After that, the precipitate remaining after the removal of the supernatant is subjected to drying by heating (or thermal depressurization), and the dried precipitate is subjected to pulverization for subsequent process of achieving uniformity in distribution of particle diameters by using a roll mill.    Patent Document 1: Japanese Patent Application Publication No. 5-87760 (JP05-087760A)